Mammography is a well-established method of breast imaging which may be used for breast cancer screening and diagnosis. Screening mammograms are preferably obtained annually for female members of the population over the age of forty, or those having a genetic risk of breast cancer. Should masses or calcifications (‘regions of interest’) be identified during a screening mammogram, the patient may require further diagnosis. Such diagnosis may involve biopsying the region of interest and analyzing excised tissue.
Various imaging modalities have historically been used during breast biopsies. The imaging modalities include ultrasound imaging, x-ray imaging and magnetic resonance imaging. Performing a breast biopsy typically involves positioning the patient, visualizing the region of interest using the imaging equipment, targeting coordinates of the region and retrieving cells or tissue from the targeted region. Cells or tissue may be retrieved in a variety of ways, including through open surgery, fine needle aspiration, core needle biopsy or vacuum assisted biopsy. Open surgery, the most invasive procedure, is generally performed by a radiologist placing a wire into the breast during visualization of the region of interest, where the wire extends into the region that is to be excised. The patient is then transferred to surgery and tissue is retrieved using the wire to locate the region of interest.
Fine needle aspiration, core needle biopsies and vacuum assisted biopsies are less invasive than open surgery, allowing cells and tissue to be obtained without the need for open surgery. All are needle biopsies, with the size of the needle, and thus the corresponding size (and number) of the biopsied samples, being differentiators. In each procedure the patient is positioned, the region of interest is visualized, the needle of the biopsy device is advanced to the target region of interest and the tissue is retrieved. Fine needle aspiration and core needle biopsy devices typically retrieve one tissue sample and their advancement to the target may be monitored using an imaging modality such as ultrasound. Vacuum assisted biopsy devices generally have larger needles and can extract multiple cores.
X-ray imaging in stereotactic mode is generally used for breast biopsies because it is desirable to visualize and target regions in a three dimensional volume. Stereotactic biopsies obtain volume information using x-ray images taken in at least two planes. The x-ray images are then processed to localize a target region of interest in three-dimensional space using the principal of parallax to determine the depth, or Z dimension, of the target region.
U.S. Patent Application 2008/0045833, filed Feb. 21, 2008 and incorporated herein by reference, describes systems and methods for using tomosynthesis for lesion localization during breast biopsy. Tomosynthesis (tomo) is a method of performing three dimensional (3D) breast x-ray imaging. It generates images of cross sectional slices through a compressed breast, and also is used to identify breast pathologies. One of the advantages of tomosynthesis is that the images are three-dimensional so that once an area of interest is identified in an image its exact 3D coordinate in the breast can be calculated or estimated, e.g. from the x, y coordinate in the image of a slice and from the z, or depth, coordinate given by the image slice depth location. Another advantage of tomosynthesis is its ability to provide high contrast visibility of objects by the suppression of images from objects at different heights in the breast. Because of its superior contrast visibility, it is expected that there will be pathologies seen on the tomo images that will not be visible using standard x-ray mammography, stereotactic devices, ultrasound or even MRI. For this reason, it is desired to develop localization methods using tomosynthesis systems that utilize tomosynthesis' natural 3D localization abilities.